This invention generally relates to the servicing of the omega seal site where a control rod drive mechanism attaches to the adapter tube of a nuclear reactor head stalk, and is specifically concerned with welding a canopy seal about the omega seal site to contain the radioactive steam released in the event of an omega seal failure.
The core of a modern nuclear reactor of the type used to generate electrical power generally includes an upper internals assembly disposed over a lower core barrel. The lower core barrel houses an array of fuel rod assemblies which generate heat as a result of a controlled fission reaction that occurs in the uranium oxide pellets present in the individual fuel rods. Water is constantly circulated from the lower core barrel through the upper internals and out through outlet nozzles provided in the walls of an upper core barrel in order to transfer the heat generated by the fuel rod assemblies to heat exchangers which ultimately convert this heat into usable, nonradioactive steam.
The upper internals assembly includes an upper core barrel arranged in tandem with the lower core barrel of the reactor. The ceiling of the upper core barrel is formed from an upper support plate. The peripheral edge of this support plate is seated around the upper edge of the upper core barrel. Both the support plate and the upper core plate which underlies it include a plurality of apertures for both conducting the stream of hot, pressurized water exiting the fuel rod assemblies to the heat exchangers, as well as for conducting control rod assemblies. Separate guide tubes are provided between apertures in both the support and core barrel plates which are aligned with each other and with one of the fuel assemblies in the lower core barrel. The purpose of these guide tubes is to align and guide the relatively long and flexible rodlets of the control rod assemblies into a particular fuel assembly.
The rate of the fission reaction taking place within the fuel rod assemblies is regulated by means of the control rod assemblies. Each of these control rod assemblies is formed from an array of stainless steel tubes containing a neutron absorbing substance, such as silver, indium or cadmium. The stainless steel tubes (known as "rodlets" in the art) are suspended from a spider-like bracket. A reciprocable drive rod is connected to the spider-like bracket for either inserting or withdrawing the rodlets of the control rod assembly deeper into or farther out of each of the fuel rod assemblies in order to modulate the amount of heat generated thereby. These reciprocable drive rods are driven by control rod drive mechanisms which may be of the electromagnetic linear motion drive type devices or hydraulic drive type devices which move the control rods in incremental steps into and out of the reactor core. Each of the control rod drive mechanisms are attached to the reactor vessel head by way of adapter tubes, with the control rod drive mechanism being sealed to the adapter tube by way of an omega-type seal (so-called because of its resemblance in cross-section to the Greek letter omega).
Because hot, radioactive primary water is contained within the control rod drive mechanism, a leak may develop during the course of operation between the control rod drive mechanism and the adapter tube due to corrosion. Any such leak will promote further corrosion and lead to greater contamination around the area of the leak due to the radioactive steam released by the leak. Presently, due to the high radiation field and lack of working space, the omega seals must be repaired manually which requires a shut down of the reactor vessel and the removal of the control rod guide mechanism.
Applicant has observed that the servicing of broken omega seals requires extensive down time of the reactor vessel, with a large amount of labor costs as well as exposure of servicing personnel to potentially harmful radiation. Clearly, there is a pressing need for a system for both efficiently and effectively servicing broken omega seals that minimizes or eliminates reactor vessel down time, which typically costs the utility over $100,000 per day in lost revenues. Ideally, the system should reduce service personnel exposure to radiation contamination and contain the radioactive steam emitted from the broken omega seal.